For people in same shoe like me;-
Since I use PKI smart card to hold my private key at me desktop machine, and testing server does not have the card. I have been forced to use "https://". Plus I am lazy.
git clone https://github.com/arduino/OpenWrt-yun.git
after git the code, the src-git also need to be changed.
nano ~/OpenWrt-yun/feeds.conf.default
#src-git packages git@github.com:arduino/openwrt-packages-yun.git
src-git packages https://github.com/arduino/openwrt-packages-yun.git